The objective of the proposed research is to describe the cellular mechanisms underlying a reflex bradycardia in the marine gastropod mollusc, Aplysia willcoxi. We have monitored heart rate in intact A. willcoxi and A. californica by a variety of techniques. In both species the heart rate slows significantly when the animal is removed from sea water and placed in air. This bradycardial response may be analogous to the oxygen-conserving reflexes of flying fishes and also of air- breathing diving mammals. In A. willcoxi a significant component of this bradycardia is centrally mediated as shown by studies in which the abdominal ganglion was removed. Kandel and his coworkers have recently described motoneurons and interneurons which excite or inhibit the heart in A. californica. We propose to use a combination of behavioral and cellular neurophysiological techniques in order to examine the central component of the bradycardial reflex in A. willcoxi. We will first determine the adequate stimulus that triggers the behavioral reflex in the intact animal. The first stage of the neural analysis will be to specify the motor end of the neural circuit controlling heart rate in A. willcoxi by examining the homologous cells to the heart motoneurons and interneurons in A. californica. The final stage of the neural analysis will be to identify the sensory cells mediating the bradycardial reflex and to determine whether these cells act at the level of the interneurons and/or the motoneurons to produce their effect. The significance of this research is that it can enhance our understanding at the cellular level of the way in which the central nervous system generates an adaptive behavioral reflex in a simple animal. Ultimately it is hoped that a simple system of this sort may serve as a model for more complex adaptive behaviors in vertebrates.